The soot suppression of acetylene diffusion flames by applying acoustic oscillation was studied experimentally. The soot emission decreased by increasing the sound pressure level and lowering the frequency of the acoustic oscillation. The efficiency of soot suppression can be related with acoustic Reynolds number, Re*. The result showed that the efficiency of soot suppression exceeded 90% in a region of Re* > 3000. The flame temperature increased by applying the acoustic oscillation. It is considered that the acoustic oscillation enhanced the mixing of the fuel gas and surrounding gas, consequently, the high flame temperature caused the re-oxidation of soot particles.